Modelling Carbon Black Matthew Celnik, Tim Totton, Abhijeet Raj, - - PowerPoint PPT Presentation

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Modelling Carbon Black Matthew Celnik, Tim Totton, Abhijeet Raj, - - PowerPoint PPT Presentation

Feb 26, 2024 278 likes •609 views

Modelling Carbon Black Matthew Celnik, Tim Totton, Abhijeet Raj, Markus Sander, Markus Kraft 09/09/09 Soot Formation Temperature Reaction Zone Burner Flame Carbon Addition Reactions Condensation of PAHs Particle Inception by PAHs

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SLIDE 1

Modelling Carbon Black

Matthew Celnik, Tim Totton, Abhijeet Raj, Markus Sander, Markus Kraft

09/09/09

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SLIDE 2

Markus Sander ms785@cam.ac.uk

Soot Formation

Burner

Reaction Zone

Temperature

Flame Condensation of PAHs Coalescense Particle Inception by PAHs Aggregation Carbon Addition Reactions Oxidation by O2 and OH,

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SLIDE 3

Markus Sander ms785@cam.ac.uk

Molecular soot structure

  • Nanoparticles (3 nm – 2 µm)
  • ‘Primary particles’ occur due to

dimerisation of PAH molecules

  • What is the critical PAH size to form a

dimer in flame conditions?

  • What is the morphology of a soot particle?
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SLIDE 4

Markus Sander ms785@cam.ac.uk

Soot model hierarchy

Quantum Chemistry (DFT) Full representation of molecules

Determine kinetic parameters

PAH Aromatic Site Model (ARS) Functional site description

PAH reactions as jump processes

Kinetic Monte-Carlo (PAH KMC) Single planar PAH simulations

Used to generate internal particle structure

Population Balance (PAH-PP) Particle ensemble modelling

Particles described by PAH-PP Model Inception, growth and coagulation

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SLIDE 5

Markus Sander ms785@cam.ac.uk

Oxidation processes in PAHs

PAH: Polyaromatic hydrocarbons

Investigated reactions:

Oxidation process: Decomposition process:

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SLIDE 6

Markus Sander ms785@cam.ac.uk

Using structural information

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SLIDE 7

Markus Sander ms785@cam.ac.uk

Oxidation rates of different site types

Units: k in cm3/(mole*s), T in K Zigzag next to zigzag (zz)

Eact=156 kJ/mole

Zigzag next to free edge (zf)

Eact=161 kJ/mole

Armchair next to free edge (af)

Eact=173 kJ/mole

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SLIDE 8

Markus Sander ms785@cam.ac.uk

PAH reactions (selection)

S1 S2 S3 S4 S5 S6 Free-edge growth Free-edge desorption 5-member ring addition 5-member ring desorption Armchair growth 5- to 6-member ring Frenklach, Wang, Violi

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Markus Sander ms785@cam.ac.uk

PAH KMC growth simulation

Seed molecule: Seed molecule: Pyrene Pyrene Growth of a PAH molecule – kinetic Monte Carlo (KMC) simulation

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Markus Sander ms785@cam.ac.uk

PAH growth mechanism

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Markus Sander ms785@cam.ac.uk

Particle Model

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Markus Sander ms785@cam.ac.uk

PAH-PP model: State Space

n: number of primary particles p of particle P C: Matrix containing the sphericity of the neighbouring primaries Cij=0 if pi and pj are not touching

Each primary particle pi is composed of m PAHs:

Structure of a particle:

) , ,..., (

1

C

n

p p P P =

) ,..., (

1 m i i

PAH PAH p p =

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Markus Sander ms785@cam.ac.uk

PAH-PP model: Data structure

Contains: [C]2,5 Contains: [C]5,6

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Markus Sander ms785@cam.ac.uk

PAH mass spectra

Monomers Dimers Monomers Dimers C2H4 - O2 flame, Pressure = 120 mbar, C/O = 1, Cold gas velocity = 54 cm/sec

  • J. Happold, H.-H. Grotheer, and M. Aigner. Rapid Commun. Mass Sp., 21:1247–1254, 2007.

Experimental Computed

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SLIDE 15

Markus Sander ms785@cam.ac.uk

Coagulation efficiency

⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + − + = 5 1100 2 exp 1 1

6 min min 3 min

M M D CE

PAH smaller

  • f

Mass PAH smaller

  • f

Diameter efficiency n Coagulatio

min min

= = = M D CE

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Markus Sander ms785@cam.ac.uk

PAH coagulation efficiency

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Markus Sander ms785@cam.ac.uk

Investigating structure

  • How do PAH molecules

form clusters?

  • How do these clusters

grow?

  • Driven by intermolecular

potentials

Alston Misquitta, Aron Cohen, Dwaipayan Chakrabarti, Mark Miller, David Wales

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Markus Sander ms785@cam.ac.uk

Basin hopping

  • Finds stable

molecular clusters by searching for minima

  • Based on potential

energy ‘landscape’

  • Uses Monte-Carlo

criterion when ‘jumping’ between minima

Energy

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SLIDE 19

Markus Sander ms785@cam.ac.uk

Investigating structure

  • How do PAH molecules

form clusters?

  • How do these clusters

grow?

  • Driven by intermolecular

potentials

Alston Misquitta, Aron Cohen, Dwaipayan Chakrabarti, Mark Miller, David Wales

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SLIDE 20

Markus Sander ms785@cam.ac.uk

Experimental comparison

A TEM-style projection

  • f

a computed cluster of 50 coronene molecules Experimental HR-TEM images of an aggregate sampled from a diesel engine. Indicated are length scales of structures within a primary particle (from Mosbach et al., 2009, Combustion and Flame).

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Markus Sander ms785@cam.ac.uk

Thank you! Please visit our website:

http://como.cheng.cam.ac.uk

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Markus Sander ms785@cam.ac.uk

DISI Engine

  • Late injection

produces stratified mixture.

  • Fuel rich regions

close to spark gap.

Image from www.engineforall.com

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Markus Sander ms785@cam.ac.uk

Soot in DISI Engine

λ = 1.0 EOI = -50 CAD ATDC Spark = -30 CAD ATDC

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Markus Sander ms785@cam.ac.uk

Soot in DISI Engine

32.6 CAD ATDC 12.6 CAD ATDC 2.6 CAD ATDC

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Markus Sander ms785@cam.ac.uk

PAH growth rates - sensitivity

  • P. Weilmünster, A. Keller, and K. H. Homann. Combust. Flame, 116:62–83, 1999.
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Markus Sander ms785@cam.ac.uk

Global minimum clusters

10 Coronene molecules 2 Coronene molecules 5 Coronenes molecules E = -94.90 kJ/mol E = -394.35 kJ/mol E = -926.42 kJ/mol

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Markus Sander ms785@cam.ac.uk

Comparison of potentials

2.5 3 3.5 4 4.5 5 5.5 6

  • 50
  • 40
  • 30
  • 20
  • 10

10 20 Dimer Separation (Å) Potential (kJ/mol) LJ Gr LJ SP LJ X W99 Gr W99 SP W99 X SAPT Gr SAPT SP SAPT X

  • Poor agreement

between L-J potential and SAPT(DFT) results

  • Good agreement

with W99 potential

Graphite (Gr) Slipped Parellel (SP) Crossed (X)

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SLIDE 28

Markus Sander ms785@cam.ac.uk

Cluster Density

  • Need to define cluster volume
  • Used scaled ‘van der Waals’ radii
  • Define Volume in terms of scaling factor, α
  • Determine ‘critical’ α to determine density
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Markus Sander ms785@cam.ac.uk

Cluster density

  • Volume varies non-linearly

with α

  • Choose critical α at minimum
  • f dV/dα (αcrit = 1.7)
  • Corresponds to point where

all intermolecular space is covered by overlapping spheres.

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Markus Sander ms785@cam.ac.uk

Varying density in our models

  • For αcrit = 1.7,

– coronene ρ = 1.1 g/cm3, pyrene ρ = 1.0 g/cm3

  • Standard soot density in models = 1.8 g/cm3
  • Density has been shown to be an important parameter
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SLIDE 31

Markus Sander ms785@cam.ac.uk

HRTEM images of soot

  • Some evidence for

different soot structures based on different fuels

  • Top: ‘graphitic’
  • Bottom: ‘amorphous’

Pictures from: Uitz, Cracknell, Jansma and Makkee, “Impact of diesel fuel composition on soot oxidation Characteristics”, SAE 2009-01-0286